Transition Metal Complex Based On Phenoliccarboxylic Ligand: Spectroscopic Inspection, DFT/TDDFT Computations, Cytotoxicity and Molecular Docking Investigation

Abstract

The study was designed to explore the synthesis, characterization, and biological evaluation of a novel Mn­(II)-3-[(3-carboxy-2-hydroxyphenyl)­methyl]-2-hydroxybenzoic complex, [Mn­(CHB–2H)·4H₂O)]·2H₂O, in comparison to its corresponding ligand (CHB). The manganese­(II) complex (Mn­(II)–CHB) was synthesized from CHB and manganese­(II) acetate in ethanol solution using the conventional method. Because most of the chemo drugs are nonselective and may cause damage to normal tissues, finding new anticancer medications or developing anticancer agents is becoming more crucial. The work’s objective was to minimize the toxic side effects of most chemo drugs and improve available chemotherapy against human liver cancer. PXRD, TEM, thermal (TGA, DSC), magnetic investigations, UV–vis, FT-IR, MS, ¹H/¹³C NMR, and elemental analysis helped to describe the structure of the ligand and the complex. The octahedral complex has been postulated through experimental and theoretical data. Different thermal (kinetic and thermodynamic) parameters were calculated using the Coats-Redfern model. The kinetic parameters revealed that the decomposition reactions of the synthesized complex followed the first-order model with the rate constant values ranging from 0.022 to 0.193 min^–1 and the activation energies were 42.18, 19.79, 55.72, and 13.20 kJmol^–1. Spherical granules were identified using the morphological analysis (TEM). The average particle sizes of CHB and Mn­(II)–CHB are 118 and 28 nm, respectively. The notable reduction in particle size indicates that the modification with Mn­(II) has changed the physical characteristics, confirming the formation of the nanocompound. XRD data reflect that the Mn­(II)–CHB complex is amorphous, while the CHB ligand is crystalline. Electronic spectra quantified the optical properties like energy gap, refractive index, optical conductivity, and penetration depth. The compounds’ band gap values (CHB = 3.20 eV and Mn­(II)–CHB = 2.12 eV) are within the semiconductor range. The structural and electronic properties of the Mn­(II)–CHB were elucidated through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The optimized high-spin sextet geometry, detailed frontier orbital analysis, and computed UV–vis spectrum reveal strong metal–ligand orbital mixing and a large HOMO–LUMO gap consistent with high kinetic stability and selective charge-transfer excitations. The examination of cytotoxicity against hepatocellular (HepG-2) cells, which are a type of human liver cancer cell, demonstrated significant growth inhibition. Based on experiment results, the CHB gave a respectable effect to prevent 50% of cell multiplication (IC₅₀ = 66.78 ± 4.76 μM). A molecular docking simulation was conducted to assess the binding mode and affinity of the target compounds toward peroxisome proliferator-activated receptors [PDB ID: 1i7i], which are associated with hepatocellular malignancies. The enhanced binding affinity of the Mn­(II) complex (−7.9 kcalmol^–1) compared to the parent ligand (−7.4 kcalmol^–1) correlates with its improved anticancer activity. The investigated assays suggest that the synthesized compounds are potent antiliver cancer agents.

1. Introduction

Phenoliccarboxylic acids, which are aromatic acid compounds, have many unique applications that are crucial building blocks for the creation of functional materials. 3-[(3-carboxy-2-hydroxyphenyl)­methyl]-2-hydroxybenzoic acid (CHB, Figure ) is a class of these compounds currently being researched, and their metal complexes are used in bioinorganic and industrial chemistry, electrochemistry, catalysis, and corrosion. − CHB (C₁₅H₁₂O₆) is essential to coordination chemistry because it contains two phenolic units that enable it to behave as a polydentate ligand. It can easily generate complexes when combines with Cu­(II), Ni­(II), Zn­(II), Mg­(II), and Ba­(II) ions. ,− While some of the phenoliccarboxylate complexes of some transition metal ions [M = Cu­(II), Ni­(II), and Zn­(II)] have been reported, ,, the nanostructure Mn­(II) complex is novel and has not been discussed before. Nevertheless, the coordination nature of the CHB ligand has not been sufficiently researched; some closely related compounds have been reported. For example, 5,5′-methylene disalicylic acid, pamoic acid, and methyl salicylate can form solid complexes with transition metals, including rare earths. − The physicochemical study of square-planar Co­(II), Ni­(II), and Cu­(II) complexes with the bis-oxime of 5,5′–methylene­(salicylaldehyde) was explained by Patel et al. According to the findings, the complexes’ thermal stability is arranged as follows: Ni­(II) > Cu­(II) > Co­(II). Regarding Mn ions, the coordination chemistry is currently of attention. Uses of manganese­(II) complexes in biology, organic synthesis of various compounds, agriculture, and many catalytic activities all contribute to their significance. − In fact, the synthesis and characterization of manganese complexes have been extensively documented and continue to be an active area of research in inorganic and bioinorganic chemistry. −

1.

Synthesis procedure of manganese complex (Mn­(II)–CHB) from the phenoliccarboxylic ligand (CHB). Note: Synthesis involves a metal center and a ligand binding to it through electron pair donation, which is then functionalized to achieve a Mn­(II)–CHB target. Mechanism includes coordination of ligand, via its donnor atoms, to a central Mn­(II) ion for generating a nano complex structure.

Theoretical computations (DFT and TD-DFT) are important tools for studying the electronic and structural properties of metal complexes, including Mn­(II) ion. ,− In this study, the computational characterization of a manganese coordination complex exhibiting a high-spin sextet multiplicity (S = 5/2). High-spin Mn­(II) species are of particular interest due to their rich electronic structures, arising from the occupation of five unpaired d-electrons in a weak or intermediate ligand field. Such configurations often confer unique magnetic, redox, and spectroscopic properties that are relevant in catalysis, bioinorganic chemistry and materials science. , To gain insights into these aspects, we employed DFT and TD-DFT methods to optimize the ground-state geometry, assess the vibrational stability, and predict the electronic absorption spectrum of the Mn­(II)–CHB complex in ethanol solvent. , These calculations not only confirm the stability of the high-spin sextet ground state but also provide a comprehensive description of the charge-transfer and ligand-centered excitations that define the compound’s photophysical behavior.

Malignant tumors are caused by abnormal proliferation and division of normal cells. Hepatocellular carcinoma (HCC) is the fifth most frequent cancer and a potentially fatal case. People with chronic viral hepatitis C (HCV) and B (HBV) infection as well as cirrhosis are more likely to suffer liver cancer in addition to excessive alcohol consumption. Signs of advanced liver cancer may include swelling and pain in the abdomen, weight loss, yellowing of the skin and also fever. The most common treatments for liver cancer are tumor ablation and chemotherapy delivered directly into the cancer. Chemotherapy is still the cornerstone of cancer treatment. The inherent lack of selectivity and serious adverse effects of traditional cytotoxic chemotherapies on healthy cells underscore the urgent need for developing safer, more selective anticancer agents. The practical implication of drug or material development is the promotion of human health and lowering the burden of disease. The discovery of new chemical compounds used in medicine provides clinicians with effective tools to treat or cure diseases that were previously unmanageable, significantly improving survival rates and patient outcomes. With regard to the literature review, several compounds like gallic acid, platinum complexes, ellagic acid, evodiamine derivatives, and 7-ethyl-10-fluoro-20-O-(cinnamic acid ester) have been tested as potential anti–liver cancer agents. Salicylic acid, a compound with a structure analogous to CHB, also showed an unquestionable anticancer effect. Currently, metal complexes as anticancer agents represent an active area of research. A series of transition metal complexes were developed by El-Boraey et al. using tetraamide macrocyclic ligand. The antitumor activity of these complexes was evaluated against human breast cancer (MCF-7) and hepatocarcinoma (HepG2) cell lines. Terpyridine-metal complexes have been shown to have therapeutic potential against human colorectal cancer. In fact, a number of physiological functions, including immunological responses against cancer, depend on manganese (Mn). Mn complexes are more stable and inert to biomolecules because of the sheltering action of ligands, which could mitigate the toxicity of Mn­(II) ions even though excessive Mn­(II) ions may cause system toxicities. On the other hand, the oxidation state of manganese enables their complexes to contribute significant roles in bioavailabilities. , Shakir et al. prepared macrocyclic complexes with the formula [MnLCl₂], and this complex was used as an anticancer agent. Imidazolium-thiohydantoin hybrids–Mn­(III), Mn­(II) of 1,4,7-triazacyclononane-1,4,7-triacetic acid, and manganese­(I) tricarbonyl,were examined as antiliver cancer. Mn­(II)-nanostructured octahedral complexes of ether ligand have been synthesized and tested for cytotoxicity against liver and breast cancer proteins. Based on the previous information where several manganese complexes were being considered, the manganese complex (Mn­(II)–CHB) was selected due to its potential properties for application. Molecular docking (MD) technique investigates the ability of ligands to bind to specific binding sites by describing the interactions between ligand molecules and proteins as receptor or target molecules. − Targeting the peroxisome proliferator-activated receptor protein, which is implicated in the pathophysiology of HCC, was the aim of the current investigation. , Actually, research on CHB or its related Mn­(II) complex has not been conducted for human liver cancers yet. The present work is unique in that it combines experimental synthesis, comprehensive spectroscopic characterization, and DFT/TD-DFT analysis of a Mn­(II)–CHB complex that has not been previously reported for hepatocellular carcinoma applications. Unlike earlier studies on Mn­(II) complexes with simple carboxylates or Schiff bases, − this research employs a phenolic-carboxylic ligand (CHB) capable of dual O,O-coordination, enhancing metal–ligand orbital mixing and stability. Moreover, the work integrates thermogravimetric kinetics, optical semiconductor characterization, and molecular docking toward the peroxisome proliferator activated receptor (PDB ID: 1i7i), linking structural features to potential biological function. These combined physicochemical and theoretical approaches highlight the distinctive advantage of the Mn­(II)–CHB system in merging semiconducting, kinetic, and biomedical properties, which have not been concurrently explored in prior Mn­(II) coordination studies. ,− ,−

The work presents a study on the synthesis, characterization, and biological evaluation of a manganese­(II) complex. It combines experimental and theoretical analyses, which could provide valuable insights into coordination chemistry and anticancer drug design. The discussion elucidates how structural modifications and metal coordination influence the physicochemical and biological properties of 3-[(3-carboxy-2-hydroxyphenyl)­methyl]-2-hydroxybenzoic acid (CHB) and its Mn­(II) complex. Specifically, it seeks to establish a structure–activity correlation between coordination geometry, optical/electronic behavior, and anticancer potential. Experimental investigations are combined with computational validation using DFT and TD-DFT methods to confirm the optimized geometry, HOMO–LUMO characteristics, and charge-transfer nature of the complex. A Cytotoxicity study was conducted to reveal the compounds’ destructive effect on human liver cancer cells. Moreover, molecular docking simulations against the peroxisome proliferator-activated receptor (PDB ID: 1i7i) are employed to rationalize the potential biological impact. This integrated experimental–computational framework enables a precise assessment of how coordination structure governs both the stability and bioactivity of the Mn­(II)–CHB complex.

2. Experimental Section

For more details, see the Supporting Information file. −

3. Results and Discussion

Several physicochemical methods were used to confirm the structural formula of the synthetic compounds. The combination of melting point, elemental analysis, and various spectroscopic techniques (FT-IR, mass, and ¹H, ¹³C NMR) confirmed the isolated ligand structure; see Supporting Information file.

3.1. Infrared Spectroscopy

Characteristic main IR bands of the CHB ligand are assigned in the Supporting Information file. When the IR spectra of the CHB and its Mn­(II) complex are compared (Table ), the complex’s bands show significant splitting. The CHB acts in a bidentate manner with Mn­(II) ion participating with one side (right or left) of the (COOH) and (OH) groups with removal of the acidic protons from them. ,, The following facts supported the previous behavior: (1) The appearance of both υ­(COO^–) and υ­(COOH) in the Mn-complex clarifies the deprotonation of one of the carboxyl groups. (2) Splitting and shifting of υ­(C–O)_phenolic at 1337 and 1369 cm^–1 points to the coordination with metal and presence of two types of phenolic (C–O) groups. Also, the shift and splitting for the carboxylic υ_asymCO (belongs to CHB) to 1616 and 1622 cm^–1 points to the presence of two types of CO indicating the existence of (COO^– and COOH) groups in the complex. (3) Bands at 3150 and 2390–2650 cm^–1 are obscured, confirming that chelation breaks the intermolecular hydrogen connection between phenolic–OH and carboxylic groups. Meanwhile, the appearance of the phenolic–OH (3200 cm^–1) and COOH (1622 cm^–1) in the complex at lower positions with weak relative intensities indicates their foundation in the proposed structure but with the formation of interamolecular H-bonding between uncoordinated phenolic OH and an adjacent carboxyl (−COOH···OH−), Figure . The existence of δ­(OH) phenolic in the complex at nearly the same position (L: 1200 → Complex: 1196 cm^–1) with weak intensity refers to one of the phenolic OH groups is still being present without coordination with metal center. The COO group bonds to the Mn­(II) ion in a monodentate fashion, as indicated by the 244 cm^–1 separation value between the asymmetric (1616 cm^–1) and symmetric bands (1372 cm^–1) (C–O) vibrations. Worthy mention, the υ_asym(COO^–) and υ_sym(COO^–) in the Co­(II), Ni­(II) and Cu­(II) carboxylate complexes show the separation value (Δυ) greater than 200 cm^–1 indicating monodentate binding of the carboxylato group. The presence of new weak bands localized at 655/593 (Mn–O_carboxylate/ Mn–O_phenolate) suggests the building of the target complex. ,,,, The Mn­(II) complex’s coordinated water is indicated by the two much weaker bands corresponding to wagging (OH_water: 617 cm^–1) and rocking (OH_water: 882 cm^–1). Stretching vibrations (υOH) at 3037–3537 cm^–1 are assigned to coordinated and crystalline water molecules in the proposed complex structure (Figure ).

1. Spectral and Magnetic Data as Well as Thermodynamic and Kinetic Parameters.

		compounds/(formula, molecular weight)
		ligand: CHB/(C15H12O6/288.26)	complex: [Mn(CHB–2H)·4H2O]·2H2O/(C15H22O12Mn/449.27)
assignment		vibration frequencies (cm–1)	observation	vibration frequencies (cm–1)	indication
υ(OH)phenolic		≈3150 (intermolecular H-bond)	obscure	∼3200 (br) (intramolecular)	breakdown of the intermolecular hydrogen bonding with formation of intramolecular H-bonding
υ(COOH)		2390–2650 (carboxylic hydroxide as an intermolecular H-bond)	obscure	1622, ∼3100 (br) (intramolecular)
δ(OH)phenolic		1200	existence with weak intensity	1196(w)	one of the phenolic OH groups is still present without coordination
υ(C–O)phenolic		1286	splitted and shifted upon complexation	1337, 1369	coordination with Mn(II) ion and presence of two types of phenolic (C–O) groups
υ (CO)		COOH: 1650	splitted and shifted upon complexation	COOH:1622, COO–:1616	deprotonation of one of the carboxyl groups
υ (Mn–O)		-	observed	593 (phenolic), 655 (carboxylate)	building of target complex
υ(OH)H2O		-	observed	3037–3537	persistence of coordinated and crystalline water molecules in the complex structure
μeff (B.M)		-		6.4
symmetry		-	octahedral, Oh
thermodynamic and kinetic parameters	phase	-	1	2	3	4
	temp (K)	-	433	697	805	1039
	k (min–1)	-	0.022	0.033	0.193	0.083
	t 1/2 (min)	-	31.62	20.75	3.59	8.38
	E a (Jmol–1)		42.18	19.79	55.72	13.20
	ΔH (Jmol–1) × 103		–3.56	–5.78	–6.64	–8.62
	ΔS (Jmol–1K–1)		–263	–340	–287	–356
	ΔG (Jmol–1) × 104		11.04	23.15	22.46	36.10

3.2. Magnetic Investigations and Electronic Spectra

Compared to the CHB ligand in its free state (see Supporting Information file), UV–Vis. spectrum of the Mn­(II) complex (Figure S5, Nujol) exhibited five bands at 331, 362, 371, 380, and 393 nm assignable to ⁶A_1g → ⁴T_1g(⁴P), ⁶A_1g → ⁴E_g(⁴D), ⁶A_1g → ⁴T_2g(⁴D), ⁶A_1g → ⁴E_g(⁴G) and ⁶A_1g → ⁴A_1g(⁴G) transitions in octahedral geometry around Mn­(II) ion (O_h symmetry). The changes in basic feature of the spectrum upon coordination confirm the formation of a Mn­(II) complex. The suggested structure derived from the electronic data was supported by the complex’s effective magnetic moments, μ_eff = 6.4 B.M (Table ). The Mn-CHB can serve as a model species for high-spin manganese complexes, which include the majority of Mn­(II) complexes. This is due to Mn­(II) having a relatively high pairing energy and only a few known structures with strong field ligands are in low-spin states, for example: [Mn­(CN)₆]^4–.

Due to the half-filled d shell’s stabilization, the metal center’s low charge, and the weak/medium–field character of the carboxylic ligand, the effective magnetic moment value (μ_eff) of the manganese­(II) complex is high. The experimental value of μ_eff could be acceptable and reliable with the assumed structure where the spin only and reported values are at 5.7–6.0. The reported μ_eff of mononuclear manganese­(II)-ditolyldithiophosphates appears in the range of 5.60–5.90 B.M., while it is 5.92 B.M in case of Mn­(II)-gibberellic acid. Magnetic measurements also revealed the formation of Mn­(II)–CHB (t_2g ³.e_g ²) complex with 5 unpaired 3d electrons. The difference between the splitting and pairing energies determines how the t_2g and e_g orbitals are occupied.

3.3. Thermogravimetric Data (TGA, DSC)

The thermal behavior of the Mn­(II)–CHB complex has been studied using TGA and DSC analysis, Figure .

2.

TGA-DSC curve of Mn­(II) complex.

TGA is done to identify the solvent molecules in the structure, and DSC curve indicates the endothermic or exothermic nature of the process, as seen in Figure . The decomposition of the complex happened in steps. In reality, loss of water molecules in the Mn complex is based on the fact that removal of water above 150 °C is caused by the expulsion of coordinated molecules, whereas water below that temperature can be regarded as crystalline water. Up to 1000 °C, four stages of the decomposition were observed in the TG curve. The removal of two molecules of crystalline water, eq , Figure , (found/calculated % = 8.3/8.0), corresponds to the first stage of weight loss (42–160 °C) which is accompanied by an endothermic effect in DSC curve.

$$[\mathrm{Mn}(\mathrm{CHB}-2\mathrm{H})·4{\mathrm{H}}_2\mathrm{O}]·2{\mathrm{H}}_2\mathrm{O}\to [\mathrm{Mn}(\mathrm{CHB}-2\mathrm{H})·4{\mathrm{H}}_2\mathrm{O}]+2{\mathrm{H}}_2\mathrm{O}$$ 1

In the second decomposition stage (201–424 °C), overlapping of exo/endothermal peaks (melting vs coordinated water loss) occurs where four coordinated water molecules are removed, eq (found/calculated % = 15.7/16.0).

$$[\mathrm{Mn}(\mathrm{CHB}-2\mathrm{H})·4{\mathrm{H}}_2\mathrm{O})]\to [\mathrm{Mn}(\mathrm{CHB}-2\mathrm{H})]+4{\mathrm{H}}_2\mathrm{O}$$ 2

The endoshape in the DSC curve is attributed to the melting process, whereas the gradual detachment of coordinated water molecules produced an exoshape. The second weight loss step represents the complex’s breakdown, where the metal-complex bond was broken.

The DSC curve revealed a mixture of exo and endothermic peaks within the third and fourth weight loss stages (436–532 and 557–766 °C) due to physical and chemical changes upon exposition to heat energy. The weight of the sample may change as a result of a number of physical processes (melting, sublimation, evaporation, etc.) and chemical changes (such as thermal breakdown, oxidation, etc.) that may occur when the sample is heated for analysis. The DSC curve indicates that the Mn­(II)–CHB starts to break down at 320 °C, peak maximum equals 327 °C (first exothermic peak). Manganese oxides are eventually being produced at the final stage by thermal disintegration.

Activation energy (E _a) and other thermodynamic parameters were computed using the first-order reaction rate equation, assuming that all phase changes were first-order reactions

$$\ln (1-x)=-\mathit{kt}$$ 3

Where x = (w _o– w_t )/(w _o– w _f), w _t is the sample weight at particular time t, the beginning weight is denoted by w _o and the final weight by w _f. A straight line for each phase is obtained by graphing ln­(1 – x) with t, indicating that the transformation is a first-order reaction. The high correlation factor (R ²) for the first-order kinetic model plot (Figure ) confirms that the decomposition is likely first-order controlled.

3.

Plot of ln­(1 - x) vs time for four phases 1–4.

Each line’s slope represented the phase’s rate constant (k) (Figure ) and the following formula was used to get the half-life (t _1/2)­

$$t_{1/2}=0.693$$ 4

Phase-specific k and t _1/2 values (phases: 1–4) are given in Table . A modified Coats and Redfern model was used to determine kinetic parameters, as indicated by the eq .

$$\ln [(-\ln (1-x)]=\ln [{\mathit{ART}}^2/\beta E_{\mathrm{a}}]-E_{\mathrm{a}}/\mathit{RT}$$ 5

Where R is the general gas constant (8.3143 Jmol^–1K^–1), A is a pre-exponential factor, β is the heating rate (10 °C/min), T is temperature (K) and E _a is an activation energy. The activation energy value was obtained by plotting graphs between ln­[−ln­(1 – x)] vs 1000/T for each phase (Figure ). The stability and breakdown sequence of the Mn­(II)–CHB complex can be inferred from the activation energies (E _a) given for the first-order thermal decomposition of the complex. The activation energy of the complex components is closely correlated with their stability. According to the E _a values (Table ), the relative stability of the decomposition stages is Stage 3 (55.72 kJmol^–1) > Stage 1 (42.18 kJmol^–1) > Stage 2 (19.79 kJmol^–1) > Stage 4 (13.20 kJmol^–1). The higher activation energy for the third stage (55.72 kJmol^–1) suggests it involves the breaking of the most robust chemical bondsthe primary coordination sphere or the core organic ligand framework. The fourth stage, with the lowest activation energy (13.20 kJmol^–1), followed by the second stage (19.79 kJmol^–1), represents the most thermally unstable portions of the Mn complex. The reason for this is most likely because these low E _a values (13.20 and 19.79 kJmol^–1) are characteristic of physicochemical processes rather than substantial chemical bond breaking. In the second and fourth weight loss stages, a combination of exo- and endothermic peaks are seen, indicating the instability of the existing components.

4.

Plot of ln­[−ln­(1 – x)] vs 1000/T of the phase.

The following thermodynamic equations were used to determine additional variables, as reported. ,

$$\mathrm{\Delta }S=R\ln (\mathit{Ah}/\mathit{KT})$$ 6

$$\mathrm{\Delta }H=E_{\mathrm{a}}-\mathit{RT}$$ 7

$$\mathrm{\Delta }G=\mathrm{\Delta }H-T\mathrm{\Delta }S$$ 8

Where ΔS = entropy changes, ΔH = enthalpy changes, ΔG = Gibbs free energy, h = Planck’s constant (6.63 × 10–34) and k = Boltzmann constant (1.38 × 10–23). Equation is crucial because it defines the change in Gibbs free energy (ΔG), a thermodynamic property that predicts the spontaneity of a process under constant temperature and pressure.

The change in entropy (ΔS) reflects the degree of disorder of the system. The degree of spontaneity of the reaction as the temperature rises depends on the sign of the entropy change. All entropy values are negative, suggesting that the decomposition yields thermally stable products with lower disorder than the reactants. A lower ΔS indicates fewer chemical and physical changes. Gibbs free energy provides information on how much energy is required to generate the products. It determines whether a specific chemical change is thermodynamically possible. According to eq , a negative ΔG indicates a spontaneous (or favorable) reaction, and a positive ΔG indicates a nonspontaneous (or unfavorable) reaction. Positive values of ΔG in Table suggest that the fluctuations or reactions are not spontaneous. Values for each phase in Table demonstrated that all phases are all nonspontaneous endothermic processes. Wothymention, The thermal stability (kinetic and thermodynamic) of N-(4–hydroxybenzaldehyde)–p-fluoroaniline and ((E)–4-[(2–(2,4–dinitrophenyl)­hydrazono)­methyl)­phenol, as well as its Mn­(II) complex, was investigated.

The kinetic data showed that the compound displayed nonspontaneous first-order decompositions. Figure shows a synthetic route for a Mn­(II) chelate complex in light of the aforementioned conclusions. The stability of a chelate complex is indicated by the size of its chelate ring. As is generally known, complexes with six-membered chelate rings are far more stable. In reality, despite its absolute insolubility, the complex mononuclear/monomer structure was proposed on the basis of the manganese content, the complex’s larg molecular size, as well as the formulation of mononuclear/monomeric structures with highly close ligands in the literature. The following mono mononuclear formulas of diphenolic/disalicylic ligands, [Ba­(C₁₅H₁₀O₆)].3H₂O, [Mg­(C₁₅H₁₀O₆)].2H₂O and [Zn­(C₁₅H₁₀O₆]·4H₂O, have been reported. The Ln­(MS)₃ complexes [Ln = La­(III), Gd­(III), Tb­(III), and Lu­(III)] were confirmed. [Cr­(SA)₂(en)]­TBA·H₂O and [Cr­(SA)­(en)₂]­Br·H₂O, (TBA = Tetrabutylammonium, SA = salicylate, en = ethylene diamine) were separated. Furthermore, Cu­(II) disalicylic complex gives a 1 ligand:1 metal stoichiometry, while Pb­(II) ions exhibit 2:1 and 1:1 ligand–metal ion stoichiometries.

3.4. Powder XRD Studies

The X-ray diffraction pattern of the Mn­(II) complex has been investigated, in comparison to the parent ligand (Supporting Information file, Figure S6), to document the crystallinity (Figure S6). The findings showed that the CHB is crystalline, indicating that its molecules are grouped in a special geometric pattern. The good crystallinity of the CHB particles is clearly demonstrated by the clear, sharp peaks. The Mn­(II)–CHB XRD pattern points to an amorphous nature, suggesting that the constituent particles are in random dispersion. A successful chelation process is supported by the manganese complex’s diffraction patterns, which differ significantly from those of the free ligand. A loss of long-range order upon complexationthe formation of an amorphous or disordered material instead of a crystalline onecan be caused by a combination of mechanistic (kinetic) and coordination (thermodynamic) factors. The complexation reaction’s mechanism and speed are related to mechanistic factors. A disordered solid is frequently the result of an increase in reaction rate and a decrease in reaction time. Because the Mn­(II) complex is formed very quickly upon heating and adding the CHB ligand, its components may not have sufficient time to organize into a stable, repeating crystalline lattice. The initial, flawed structures that form upon very fast complexation are unable to reorganize into an orderly pattern. On the other hand, using uncoordinated ethanol as a solvent accompanied by its high concentration, may induce rapid precipitation and lead to an amorphous product. The impact of coordination factors can be explained based on the formation of a regularly repeating, crystalline network that can be inhibited by the bulky CHB ligand present in the complex structure (Figure ). The steric bulk causes frustration in the lattice, forcing a nonperiodic, disordered arrangement to minimize repulsion between the ligands. Furthermore, the generated Mn­(II) complex consists of a mixture of ligands (CHB and H₂O) and this might not be well-suited for a repeating structure. Where the use of nonsymmetrical linkers can inhibit the formation of a uniform structure. The Ni­(II) Schiff base complex [Ni­(L).(H₂O).Cl], where HL = 2-((pyridin-3-ylmethylene)­amino)­phenol], was synthesized using the same reflux process as this work, resulting in an amorphous particle structure.

It has been reported that the amorphous state formation is induced by the polymorphism phenomenon, which is discovered for the Ni­(II) complexes and implies the existence of multiple conformers. On the other hand, the PXRD experiment was conducted for [Ni_1/2(pam)_1/2·(py)₂·(H₂O)₂] _n and {[Ni­(PA)·(bpp)·(H₂O)₂].DMF} _n , (py = pyridine, H₂PA = pamoic acid, and bpp = 1,3-bi­(4-pyridyl)­propane). The experimental patterns’ diffraction peaks and the simulated patterns (generated from single-crystal diffraction data) closely matched indicating the compounds’ good phase purity. , According to the Debye–Scherrer equation:

$$D=0.89\lambda /\beta \cos \theta$$ 9

where λ is the X-ray wavelength (1.54 Å for Cu Kα radiation), β is the full width at half-maximum, and θ is the peak position, the CHB’s crystallite size was determined to be 14.4 nm. The Scherrer equation could not be applied to Mn­(II)–CHB due to its amorphous form. The amorphous complex’s melting temperature is not clearly defined since it lacks an ordered atomic structure. Therefore, the amorphous complex will have practical properties that allow it to be developed into usable shapes when heated (as utilized in the production of bendable and flexible electronics). Indeed, advances in materials science and medicine discovery have been facilitated by the special properties of amorphous solids. Consequently, resistant coatings and medications with enhanced bioavailabilitythe extent to which the active drug reaches the site of action by entering the systemic circulationcan be made using the special features of amorphous Mn­(II)–CHB. Depending on the unique characteristics of the amorphous material, such as surface energy, it can change cell interactions like how cells spread, adhere, and proliferate on a surface.

3.5. Optical Characteristics

If some electrons travel from the valence band to the conduction band, current can flow through the crystal lattice. Thus, a solid’s band gap is a crucial factor affecting its electrical conductivity (E _g). Indirect band gap energy (influences a solid’s capacity to absorb light) of the produced compounds due to electron-photon-phonon interaction was measured. The CHB and Mn­(II)–CHB compounds were found to exhibit E _g values of 3.20 and 2.12 eV, respectively (Figure S7). The reduction in the optical band gap from 3.20 eV (ligand CHB) to 2.12 eV (Mn­(II)–CHB) has significantly impacted the biological behavior and anticancer potential. This change can enhance the chemical reactivity and bioactivity of the Mn complex. A narrower band gap (2.12 eV) indicates that less energy is needed for electrons to travel from the valence band to the conduction band. This increased ease of electron transfer often correlates with higher chemical reactivity, making the complex more likely to interact with biological targets such as proteins or DNA. Research on similar compounds suggests that lower energy gaps may indicate increased bioactivity and a greater potential for drugs to effectively bind with cancer-related receptors. In comparison with reported results, the E _g values of Cu­(II), Zn­(II), Co­(II) and Ni­(II)–iminooxime complexes revealed optical direct transition with band gaps of 3.8–4.1 suggesting that they may act like semiconducting materials. İsmet et al. calculated the optical band gap (E _g) of the Schiff base oligomer, 2-[(4-chlorophenyl)­iminomethylene]­phenol and its metal complexes [M = Cu­(II), Ni­(II), and Co­(II)], and it was found to be between 2.26 and 3.18 eV. Furthermore, the optical band gaps of N,N′-bis­(salicylidene)­ethylenediamine organic ligand, commonly called Salen (3.09 eV) in addition to the binuclear Cu­(II)-Salen (3.07 eV) and heteronuclear Ni­(II)/Pb­(II)-Salen (3.14 eV) complexes support the semiconductor behavior of Mn complex. , While Mn­(II)–CHB conduction is nearly equal to that of GaP (E _g = 1.74) and Cu₂O (E _g = 2.1), the conductivity of the ligand is close to that of GaN (E _g = 3.4) and ZnO (E _g = 3.37). , Worthy mentioned, the above-known semiconductors (GaP, Cu₂O, GaN, ZnO) are promising materials for electronic applications. Similarity of the band gap values with above-mentioned materials makes the prepared compounds potentially useful as transistors to regulate or control current or voltage and integrated chips. Many digital consumer devices in everyday life, including digital cameras, smartphones/mobile phones, televisions, refrigerators, washing machines, and LED lights, use semiconductors. The manganese ion tends to enhance ligand electron mobilization during the complexation process by accepting electrons into its unoccupied outer shell. This characteristic is why CHB exhibits a higher E _g value compared to its Mn­(II) complex. The smaller band gap allows for easier electronic transitions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy states. This suggests that the Mn­(II)–CHB complex will exhibit better electrical conductivity compared to the free CHB ligand. In fact, a material’s potential for both biomedical and catalytic applications is strongly correlated with its Eg value, mainly because it affects its electrical and optical characteristics as well as its reactivity. The band gap is a critical indicator for the performance of semiconductor-based catalysts, particularly in photocatalysis. Light can be absorbed by materials with certain band gap energies to produce electron–hole pairs. Materials with narrower band gaps (≤2.4 eV) can absorb visible light of the solar spectrum and this leads to higher solar-to-chemical energy conversion efficiency (e.g., hydrogen production or pollutant degradation). In contrast, materials with wider band gaps (>2.4 eV) absorb only ultraviolet (UV) light. This absorption often leads to the generation of photogenerated charges with higher kinetic energy, which can initiate redox processes. For example, these processes can result in the formation of highly reactive hydroxyl radicals, which are effective for pollutant destruction. In biomedical applications, the E _g value is important for the design of materials used in biosensing, imaging, and especially nanomedicine. Most naturally derived biomaterials tend to be insulators with large E _g values (>2.5 eV), whereas functional biomedical electronic devices often require semiconductor properties (E _g between 1 and 2.5 eV).

For optical applications, the refractive index is a crucial characteristic since it regulates the speed at which light travels through the materials being studied. It is necessary to recognize optical refractive indices in order to operate optical devices like switches, modulators, and filters. As well, one important physical characteristic that is widely used in chemistry to assess purity is the refractive index. The following relationship can be used to express the compounds’ refractive index values (n)­

$$n=\frac{(1+\sqrt{R})}{(1-\sqrt{R})}$$ 10

where R is the estimated normal reflectance as calculated using the formula below. ,

$$R(\mathrm{reflectance})+T(\mathrm{transmittance})+A(\mathrm{absorption})=1$$ 11

Figure S8 illustrates how the refractive index (n) varies with wavelength (λ, nm). It has been noted that the refractive index changes as the incoming light beam’s wavelength changes due to specific interactions between photons and electrons. The figure also illustrates how complexation alters the refractive index of the free ligand, where chelation results in a change in its n values. It is clear that the refractive index of the CHB ligand is higher than that of the related Mn­(II) complex, with the exception of around λ = 304–366 nm. Indeed, the dependence of the refractive index on the wavelength for σ-MnO₂ thin films, dioxime ligand and copper-dioxime has been emphasized and discussed. , It has been found that the refractive index decreases with wavelength.

The optical conductivity (σ_opt) of the compounds has been determined as

$${\sigma }_{\mathrm{opt}}=\mathit{nk}\upsilon =\mathit{nck}/\lambda$$ 12

Where n, c, k, υ and λ are the refractive index, velocity of the light, extinction coefficient (αλ/4π), frequency and wavelength, respectively. Figure S9 displays the relationship between σ_opt and photon energy (hυ) for the complex and its corresponding ligand. The figures clearly show how increasing the photon energy altered the σ_opt magnitudes for the separated compounds, particularly in the 2.9–4 eV range. The optical conductivity of Mn­(II)–CHB is obviously more commonly higher than that of CHB. Notably, Paul et al. reported the σ_opt change for Zn-incorporated TiO₂ as a function of hv. It was carefully noted that the σ_opt value increases with photon energy. The rising optical conductivity observed experimentally in the 2.9–4.0 eV window correlates with TD-DFT predicted low-energy excitations. In particular, weak/forbidden transitions computed near 375–315 nm likely contribute to the optical-conductivity increase in the 2.9–4.0 eV range. Meanwhile, the stronger calculated bands at 284–261 nm correspond to the rise in optical conductivity at higher energies. The TD-DFT assignment (π → π*and mixed π → d/LMCT) therefore rationalizes the experimental optical-conductivity behavior, ligand-centered π → π* excitations and LMCT transitions supply the oscillator strength that increases optical absorption and thus optical conductivity at these photon energies.

Electromagnetic radiation can either instantly vanish in a material or penetrate it very deeply, depending on the properties of the substance and the wavelength of the radiation. The depth to which light may penetrate a compound, also known as skin depth, is reached when the incident radiation intensity, passing through the material, drops to about 37% of its initial value. The penetration depth of light (δ _p ) through the compounds under investigation has been determined using the relationship

$${\delta }_{\mathrm{p}}=\lambda /4\pi k$$ 13

where k and λ are the extinction coefficient and wavelength, respectively. Figure S10 shows the δ_p as a function of the hν variation. Notably, up to a photon energy of 3.3 eV, more penetration is made by the free CHB ligand than the corresponding Mn­(II) complex; above that, the behavior is typically reflected.

3.6. Studies on Morphology (TEM)

To verify the morphological features, a transmission electron microscopy (TEM) examination for CHB and Mn­(II)–CHB was performed and some descriptive statistics are shown (Figure a,b). The particle size histogram was employed. The average particle size was determined by the peak center of the Gaussian peak that is fitted to the histograms. The CHB and Mn­(II)–CHB average particle sizes are 118 and 28 nm, respectively. The particles of the samples appeared heterogeneous with a single regular form and the ligand and complex micrographs both have regular spherical shapes. The CHB and Mn­(II)–CHB average particle sizes are 118 and 28 nm, respectively. The TEM images provided confirmation of the complex’s nanostructure in relation to the ligand.

5.

Morphological pictures, histogram and descriptive statistics of (a) CHB and (b) Mn­(II)–CHB. Particle size, nm (CHB): mean = 118, standard deviation = 24.12, minimum = 76 and maximum = 127; particle size, nm (Mn­(II)–CHB): mean = 28, standard deviation = 9.85, minimum = 11 and maximum = 49.

The drastic reduction in particle size from the ligand (∼118 nm) to its metal coordination complex (∼28 nm) is driven by the transition from loosely associated molecular aggregates to highly ordered, compact coordination networks. The mechanistic reasoning for this morphological change includes: (1) Change of molecular preorganization and packing. The free CHB ligand typically exists in a relatively flexible or extended conformation, often stabilized by loose van der Waals packing or internal hydrogen bonding, leading to larger, more irregular aggregates (∼118 nm). Upon coordination, the donor atoms of the CHB ligand are anchored into a stable, tightly packed octahedron by the Mn­(II) ion, which functions as a central node. (2) Chelation and intramolecular folding. As the CHB is multidentate, coordination causes the formation of stable ring structures around the Mn center. This process compels the ligand to “wrap” or fold around the metal ion, transforming a potentially extended organic molecule into a more globular and compact 3D conformation. By pinning the functional groups (OH, COOH) to a central metal atom, the internal flexibility of the CHB ligand is lost. This results in a decreased hydrodynamic radius and a noticeably smaller observable particle size. (3) Nucleation kinetics. Compared with the free CHB ligand, metal ions act as “seeds” (heterogeneous nucleation sites) in a solution, increasing the number of starting points for particle formation. This process results in many small particles rather than a few large ones.

Indeed, the transformation of ligand particles into zero-dimensional (0-D) nanoparticles due to chelation could be considered a successful outcome. The potential use of nanoparticles (NPs) as a tool for innovative solutions is becoming more and more popular. This is because of their distinct and novel characteristics, which set them apart from their bulk counterparts. A change in the size or shape for Mn­(II) complex may evidently influence its physical and biological properties. Nanosize (1–100 nm) and high surface area for Mn­(II)–CHB suggests that it possess highly specific and distinct characteristics. It may be used in nanotechnology, medicine, environmental remediation and quantum dots particularly its band gap (E _g) lies within the semiconductor range. Recently, the use of nanomaterials as sensors and medication delivery agents has become more. Nano compounds are commonly found in the food sector, powders, toothpaste, detergents, paints and cosmetics. Antimicrobial products, water disinfectants, textiles, diagnostic biosensors, imaging probes and conductive inks all contain nanoparticles (AgNPs).

3.7. Theoretical Calculation

The optimized geometry of the synthesized Mn­(II)–CHB complex was determined at the B3LYP/6–31G­(d,p) level, employing the LANL2DZ effective core potential for manganese in ethanol (Figure  ). Figure compiles the computed bond lengths, bond angles, and dihedral angles. Within the CHB ligand framework, C–C distances are 1.39–1.43 Å typical of aromatic π-systems and indicative of robust conjugation. The Mn–O coordination bonds to O31 and O35 measure 2.05 and 2.44 Å, respectively, in line with high-spin Mn­(II) octahedral complexes and confirming secure metal–ligand attachment. Around the metal center, angles such as O31–Mn32–O35 at 95.3° deviate modestly from the ideal 90°, reflecting slight distortion imposed by steric and electronic demands of the chelating ligand. Finally, dihedral angles, for example, −145.3° for C4–C3–C16–C19 reveal that portions of the framework are nonplanar, a feature that can modulate orbital overlap and hence the complex’s electronic behavior.

6.

Frontier orbitals and structure optimization of the Mn­(II)–CHB complex.

As seen in Figure , Frontier molecular orbital (FMO) visualizations constructed from HOMO and LUMO offer a powerful theoretical framework for assessing a compound’s chemical and kinetic stability. − By examining where the HOMO and LUMO reside, one can identify the principal electron-donating and electron-accepting regions of the molecule. The energy difference between these two orbitals, often called the energy gap (E _g), serves as a key metric for predicting a system’s stability, reactivity, and resistance to polarization. The HOMO electron density is delocalized across the Mn center and the aromatic ring that bears the coordinating C–O donor. This mixing of Mn d character with the adjacent π-system indicates strong metal–ligand orbital overlap and establishes the ligand as the primary electron donor. In contrast, the LUMO is localized on the distal aromatic ring that does not coordinate directly to Mn. Its electron density is confined to that ring’s π* orbitals, with essentially no manganese d contribution. This clearly designates the remote aryl moiety as the principal electron acceptor, and predicts that reduction or excited-state charge transfer will occur into that ring rather than onto the metal. In the Mn­(II)–CHB complex, the computed gap is about 3.86 eV. A wide energy gap correlates with significant chemical “hardness”, meaning the electron density is reluctant to deform under external perturbations. This confers additional stability against weak electrophilic or nucleophilic attacks.

However, the HOMO–LUMO gap alone provides a limited measure of chemical or biological stability, as it reflects primarily electronic hardness and does not explicitly include thermodynamic, environmental, or kinetic effects. Nevertheless, a large frontier orbital gap generally signifies low chemical softness, reducing the likelihood of facile redox events or rapid ligand substitution under physiological conditions and thereby enhancing resistance to premature decomposition or off-target reactions. In parallel, the pronounced mixing between Mn­(II) d-orbitals and ligand π-systems indicates strong metal–ligand covalence, which increases kinetic inertness and enables spatial tuning of the frontier orbitals toward biologically relevant binding regions. Together, these features support a balance between stability and controlled electronic activation, a combination that is favorable for selective anticancer activity while minimizing nonspecific cytotoxicity. Experimental validation through solubility, redox, kinetic, and in-cell studies remains essential to confirm these predictions. In Figure , Natural Bond Orbital (NBO) charges for the Mn­(II)–CHB complex show the partitioning of electron density between the metal center and the ligand framework. The Mn center carries a NBO charge of +1.205 e, consistent with a formally positive oxidation state but reduced from the formal +2 value by electron donation from the ligands. This partial charge attenuation is a result of back-donation from filled ligand orbitals into the manganese 3d manifold. This process reduces the effective positive charge on the Mn center and enhances the strength of the metal–ligand bonds.

7.

Charges derived from NBO and electrostatic potential map (ESP) of Mn­(II)–CHB complex.

The coordinating oxygen atoms carry negative NBO charges, and those with the largest negative values identify the sites of strongest metal–ligand interaction.

MEP analysis is an invaluable tool for visualizing how electronic charge is distributed across the Mn­(II)–CHB molecule and for understanding how this distribution governs intermolecular interactions and chemical reactivity. , In Figure , the MEP surface distinctly highlights negative electrostatic potential regions localized around the oxygen donor atoms that coordinate to the Mn center. These deep red zones correspond to regions of high electron density, reflecting the strong σ-donation of the oxygen lone pairs into the empty Mn­(II) d-orbitals. Such localized negative potential makes these oxygen atoms prime sites for forming hydrogen bonds or interacting with electrophilic species in solution. In contrast, the areas surrounding the Mn­(II) ion itself appear in blue or lighter shades. This indicates a positive electrostatic potential arising from the partial depletion of electron density by the metal’s high effective charge. This positive potential marks the metal center as a likely site for nucleophilic attack or for participating in polar interactions with counterions or solvent molecules.

3.7.1. Spin State and Electronic Configuration

Manganese­(II) in an octahedral environment adopts a high-spin d⁵ configuration, formally t _2g e_g giving a total spin S = 5/2 (spin multiplicity = 6). This is corroborated by the calculated ⟨S ²⟩ ≈ 8.75 (→ S ≈ 2.5, n = 5 unpaired electrons) and the large Mulliken spin density on Mn (4.79 e^–) in your UB3LYP calculation. The absence of significant spin contamination after annihilation ⟨S ²⟩ = 8.75 confirms that the sextet is the appropriate ground state. Experimentally, high-spin Mn­(II) complexes typically show spin-only moments

$${\mu }_{\mathrm{cal}}=\sqrt{n(n+2)}=5.92{\mu }_{\mathrm{B}}$$ 14

Evans’ method measurements on related MnSL₂ complexes yielded μ_eff = 5.25–5.55 μ_B over 263–321 K, slightly below the spin-only value due to orbital contributions and covalence. For Mn­(II)–CHB, the calculated unpaired count (n = 5) predicts μ_spin‑only ≈ 5.92 μ_B. The theoretical prediction often yields slightly lower μ _eff due to the assumption of perfect quenching and neglect of dynamic contributions, the experimentally determined moment can appear higher because of unquenched orbital angular momentum, spin–orbit coupling, and thermal excitation, all of which add to the effective paramagnetism of the complex. The experimentally observed magnetic moment of 6.4 B.M is in excellent agreement with the DFT predicted high-spin sextet configuration (S = 5/2, ⟨S ²⟩ ≈ 8.75). Both results confirm that the Mn­(II) center retains five unpaired 3d electrons (t₂g³eg²), characteristic of a weak-field octahedral environment. This quantitative consistency between μ_eff and the computed spin density (4.79 e^– on Mn) validates the theoretical description and supports the assignment of a high-spin octahedral Mn­(II) ground state.

TD-DFT simulations were conducted at the B3LYP/6–31G­(d,p) level (with LANL2DZ on Mn), incorporating solvent effects via the PCM model in ethanol. The theoretical UV–vis spectrum revealed both strong and weak transitions, aligning well with experimental observations in the 228.5–284.2 nm window. As summarized in Table  , notable calculated bands at 284.2 nm (f = 0.0026), 277.6 nm (f = 0.1329), and 270.9 nm (f = 0.0210) were attributed to π → π* and mixed π → d transitions, involving significant orbital overlap between the ligand and Mn center. Higher intensity transitions such as at 260.8 nm (f = 0.1295) and 228.5 nm (f = 0.1937) further supported strong ligand-based excitations with minor metal involvement. Also, the TD-DFT results predicted additional electronic transitions at 375, 374, 344, and 315 nm, but all with oscillator strength values of zero, indicating that these transitions are formally forbidden under electric dipole selection rules. Despite their lack of intensity in theoretical spectra, their presence is noteworthy as they likely correspond to weak, spin-forbidden or symmetry-restricted transitions that may gain intensity in experimental spectra due to vibronic coupling, spin–orbit interactions, or ligand field asymmetry. Their approximate alignment with the experimentally observed bands at 371–389 nm supports this interpretation. Therefore, while these transitions are theoretically inactive, their wavelengths provide insight into multiconfigurational and weakly allowed excitations within the Mn­(II)–CHB system. Overall, the combined experimental and computational data underscore the substantial electronic reorganization upon complexation. In addition, the findings reveal a nuanced interplay between ligand π-systems and Mn-centered d orbitals, characteristic of LMCT and d–d transitions in high-spin octahedral Mn­(II) complexes. Converting the computed wavelengths to photon energies shows that the TD-DFT excitations cluster into a high-energy group (≈228–284 nm; strong π → π* and mixed π → d transitions) and a low-energy group (≈315–375 nm) that are formally weak or symmetry/spin-forbidden in the electric-dipole approximation. The experimentally observed ligand π → π* bands (≈280–331 nm) match the high-energy TD-DFT bands, while the experimental low-energy absorptions centered at ≈331–389 nm (and the LMCT at ≈411 nm) align with the computed weak transitions near 375–315 nm. The low calculated oscillator strengths explain why those excitations are formally forbidden in gas-phase TD-DFT yet appear in the measured spectrum.

2. UV Data of Mn­(II)–CHB Complex.

no.	energy (cm–1)	wavelength (nm)	osc. strength	major contribs
5	35186.2	284.2	0.0026	HOMO–2(A) → LUMO + 3(A) (17%), HOMO(A) → LUMO + 6(A) (12%), HOMO–1(B) → LUMO + 2(B) (13%), HOMO(B) → LUMO + 6(B) (10%)
7	36018.5	277.6	0.1329	HOMO(A) → LUMO(A) (21%), HOMO(A) → LUMO + 3(A) (21%), HOMO(B) → LUMO(B) (10%), HOMO(B) → LUMO + 2(B) (17%)
8	36912.2	270.9	0.021	HOMO(A) → LUMO(A) (61%), HOMO(B) → LUMO(B) (12%)
9	37409.9	267.3	0.0353	HOMO(B) → LUMO(B) (68%)
10	38343.1	260.8	0.1295	HOMO–3(A) → LUMO(A) (34%), HOMO–2(B) → LUMO(B) (36%)
20	43766.4	228.5	0.1937	HOMO–6(A) → LUMO(A) (13%), HOMO–1(A) → LUMO(A) (19%), HOMO–5(B) → LUMO(B) (12%)

3.8. Cytotoxic Assay

The cytotoxic evaluation was performed as the chelating agents (like CHB ligand) have many biological applications. The CHB ligand was examined against human HepG-2 liver cancer cells. However, the complex’s insolubility in solvents prevented Mn­(II)–CHB from being evaluated. The complex’s insoluble behavior in polar organic solvents can be attributed to several factors, including: (1) The synthesizing complex, which contains both organic and inorganic metal components, is neutral rather than ionic. (2) The complex’s large molecular size. (3) Furthermore, the uncoordinated OH and COOH are connected by a strong hydrogen bond, generating a five-membered ring that is stable and has little strain (Figure ). Interestingly, the well-known red nickel bis­(dimethylglyoximate) complex with the formula Ni­[ONC­(CH₃)­C­(CH₃)­NOH]₂ is insoluble in common organic solvents despite containing two OH groups. In this complex, the free OH groups are joined through hydrogen bonds. Frequently, two strategies might lead to improving the complex solubility. (1) Adding polar functional groups like alcohols and amines. (2) Altering the nature of the complex. Conversion to an anionic or cationic complex with perchlorate or nitrate counterion may dissolve well. Figure displays the results on cell survival. The ligand concentration that inhibits half of cell proliferation (IC₅₀) for HepG-2 liver cells was found to be 66.78 ± 4.76 μM. Worthy mention, the IC₅₀ value of cis-platin in HepG2 cells is 25.5 μM. Assuming the medication resistance and side effects of cis-platin, CHB ligand has an acceptable impact. Hassan et al. reported the cytotoxic effects of bis­(2-ethylhexyl) phthalate, oleanolic acid, and lauric acid on HepG-2 cells. The IC₅₀ values were found to be 56.46, 31.94 and 83.80, respectively. The inhibition of CHB may actually be caused by the contribution of a number of factors, such as (1) ligand-protein interactions within the active site, including H-bonding, covalent, van der Waals, and hydrophobic (-staking); (2) the CHB ligand’s reaction with metal ions in the cell fluid, which may stop cell functions; and (3) chelation of CHB with metal of metallo-enzymes of cancer cells. This inhibits several crucial enzymatic functions and causes the malignant cells to die. The impact of CHB can be considered an expected outcome because its structure closely resembles that of a naturally occurring, nontoxic and anticancer salicylic acid. ,

8.

Inhibitory action of CHB ligand against hepatocellular (HepG-2) carcinoma cells. The graphic in Figure depicts the development of liver cancer.

3.9. Molecular Docking Simulation

The most frequent causes of therapy failure for HCC are adverse drug effects and delayed detection brought on by the disease’s complicated gene network. The widely used platinum-based chemotherapy drugs are effective only for limited types of tumors. Moreover, there are many serious side effects in the course of medical use. In addition, the inherent and acquired resistance of platinum-based chemotherapy drugs can also weaken the impact of platinum drugs. Therefore, there is a need to explore new antitumor metal drugs. By preparing the compounds, we aim to mitigate the impact of drug resistance, minimize toxic side effects, and improve available chemotherapy. The investigation of the binding power of the isolated compounds to target protein was conducted through the utilization of a molecular docking strategy. The binding affinity (that describes the strength of binding with the receptor) and main bonding interactions between the compounds and amino acid residues of the protein were given.

The results of the molecular docking investigation for the researched receptor are displayed in Figure and Table S1. It is generally believed that molecules and proteins have high docking activity if the docking score is greater than 5 and strong docking activity if the docking result is greater than 7. Regarding the 1i7i, which is linked to HCC, the disalicylic ligand (CHB) has a binding affinity of −7.4 kcalmol^–1. The ligand coupled with the polar, uncharged arginine and leucine amino acids (ARG-288, LEU-228), Figure a. It became apparent that the oxygen of the phenolic–OH in the CHB ligand and the aforementioned amino acids formed a normal H-bond. Matching CHB’s binding score (−7.4 kcalmol^–1) with its experimental result (IC₅₀ = 66.78 ± 4.76 μM) may initially indicate that the CHB-target link inhibits liver cancer. However, it is crucial to note that cytotoxicity in a biological system is impacted by a variety of processes that go beyond simple binding affinity to a particular target, such as metabolism, reactions with metal ions in the cell fluid, and chelation with metallo-enzymes, all of which may not be fully predicted by docking alone. As a result of not being able to dissolve the Mn­(II) complex, molecular docking simulation was carried out to illustrate its effect as an alternative to cytotoxicity. The smaller size of nanomaterials can quickly pass through physiological barriers and reach the circulatory system, lymphatic system, human tissues, and organs. Therefore, molecular docking simulation was carried out to investigate the nano Mn­(II) complex. With an improved binding affinity of – 7.9 kcalmol^–1, the target Mn­(II) complex connected with amino acids of protein through: the H of coordinated water (proline = PRO-426, bridged serine = SER-428, glutamine = GLN-430, lysine acid = LYS-422) and 2 O of coordinated carboxylate (phenylalanine = PHE-432, leucine acid = LEU-431), Figure b. One carboxylate and three coordinated water molecules in the manganese complex formed H-bonds with protein.

9.

Interaction of CHB (a) and Mn­(II)–CHB complex (b) with a 1i7i receptor. Binding score: CHB = – 7.4 and Mn­(II)–CHB = – 7.9 kcalmol^–1.

In a biological context, a more negative score indicates a stronger predicted interaction. The difference of 0.5 kJmol^–1 in docking scores indicates that the Mn­(II) complex may bind more firmly (−7.9 vs −7.4 kJmol^–1) than the ligand. To validate that the stronger docking affinity leads to increased anticancer activity, further computational studies are recommended. Computational proposals include molecular dynamics (MD) simulations to assess the stability of the Mn­(II) complex-PPAR (1i7i) interaction over time, as well as MM-GBSA/PBSA methods to calculate binding free energies from MD trajectories. Despite the insolubility of the complex, it can be used in the field of treatment. The nano feature may enable the target Mn­(II)–CHB to enter the body through ingestion, inhalation, skin penetration, and blood flow, just like inorganic nanoparticles (NPs). On the other hand, commercial application of nanoparticles in medicine is primarily focused on medication delivery to treat diseases like cancer with unparalleled precision. Therefore, the nanoscale nature of the complex and its ability to treat liver cancer, as is evident from its binding score, make it possible to benefit from the Mn-complex in the field of medicine, not only in treating cancer but also in drug delivery. It should be highlighted that, regardless of the docking assay results, experimental validation (for example, enzyme inhibition studies, MD simulations) is required to corroborate docking data. Further biological research can be conducted in order to benefit from the prepared compounds in medicine. It was reported that lauric acid, oleanolic acid, and bis­(2-ethylhexyl) phthalate displayed good interactions with the active-site residues of human peroxisome proliferator-activated receptors (PDB ID 1i7i). On the other hand, four γ oryzanol compounds were docked on human-ppar (PDB ID 1i7i) giving the best docking scores (−7.9–8.7). Worthy of mention, Mn-pyridinecarboxaldimine complexes synthesized by Vechalapu et al., exhibited a good inhibitory against liver cancer cells.

Ultimately, it is believed that modify the ligand and its metal complex may enhance anticancer potency. Various strategies can be employed to implement these modifications, focusing on improving cellular absorption and selectivity. (1) Introduce bulky hydrophobic groups (e.g., triphenylphosphine) to increase cellular membrane permeability and absorption. (2) Conjugate the ligand and complex to delocalized lipophilic cations such as triphenylphosphonium (TPP) to target the mitochondria precisely, exploiting their higher transmembrane potential in cancer cells. (3) Design manganese complexes with higher oxidation states (such as Mn­(VII) rather than Mn­(II)) that are reduced to active cytotoxic forms in the acidic or reductive tumor microenvironment. (4) Prepare heterobimetallic complexes (e.g., Mn–Ru or Mn–As) to combine distinct mechanisms of actionsuch as DNA binding and protein inhibitionin a single molecule. (5) Improving the solubility of the samples by adding polar functional groups and altering the nature of the complex to be anionic or cationic. The above modification can be tested chemically and biologically via some methodologies. According to the chemical characterization, synthesis, and geometry of the chemical compounds can be confirmed using NMR, IR, UV–vis, TGA and X-ray crystallography. Moreover, the density functional theory can be employed to calculate HOMO–LUMO energy gaps, predicting biological reactivity and target interactions. On the other hand, in vitro potency (cytotoxicity) might be used for the biological assessment. MTT/SRB and colony formation assays can contribute to determining the selectivity index (SI) and assessing the long-term proliferative ability of treated cells, respectively. Molecular docking (MD) simulation and molecular mechanics/Poisson–Boltzmann surface area (MM/PBSA) analysis can confirm that modifications to the ligand and the complex enhance binding affinity to the target protein.

4. Conclusions

3-[(3-carboxy-2-hydroxyphenyl)­methyl]-2-hydroxybenzoic acid (CHB) can be prepared readily by condensing salicylic acid and formaldehyde in the presence of sulfuric acid as a condensation catalyst and a yield over 92% can be obtained. Different powerful measurements such as elemental analyses, UV–Vis, FT-IR, MS, ¹H/¹³C–NMR, thermal (TGA, DSC), PXRD, TEM and magnetic measurements can be used to describe the CHB ligand and its bivalent manganese complex, Mn­(II)–CHB. Comparing theoretical and experimental melting points and elemental percentages (C, H, and M %) confirmed the purity of the ligand and showed that the composition of the isolated compounds matches well with the suggested formulas. It was found that the complex is mononuclear and the metal:ligand ratio is 1:1. The ligand acts as a bidentate chelate. The IR data support the coordination of manganese­(II) ions to OH and COOH groups of CHB and the proposed structure for the complex. The octahedral geometry (O_h symmetry) of the Mn­(II)–CHB complex was assumed based on the electronic absorption and magnetic data. The thermal results showed that the complex contains four coordinated water molecules and two molecules of water of hydration. Thermal (kinetic and thermodynamic) parameters were determined using the Coats and Redfern model. The obtained results proved that all the transformations followed first-order reactions. Thermodynamics parameters results of the complex showed that the Gibb’s free energy (ΔG) was positive while the obtained enthalpy (ΔH) was positive, indicating that, the decomposition processes are nonspontaneous endothermic reactions. The computational study confirms that the complex adopts an octahedral geometry with robust ligand conjugation and well-defined donor–acceptor orbital separation. The combination of a high-spin d⁵ configuration, strong π → π* absorptions, and localized LMCT transitions underscores its potential stability and tunable photophysical properties in polar media. The amorphous property of the Mn complex is well suited to designing flexible and bending electronics; in addition it can change cell interactions. The TEM images provided confirmation of the complex’s nanostructure that could be used in nanotechnology and medicine. The CHB and Mn­(II)–CHB showed average particle sizes at 118 and 28 nm, respectively. The significant reduction in particle size suggests that the physical characteristics were altered by the Mn­(II) coordination. Complexation of ligand with Mn­(II) causes reduction of the optical band gap E _g from 3.20 to 2.12 eV. It has been found that the refractive index (n) values differ after chelation. The optical conductivity as well as penetration depth of the compounds changes in response to changes in photon energy. The optical properties indicate the generation of prospective materials that could be useful in a wide range of applications, including semiconductors and electronic devices. The CHB was tested on human liver cancer cells (HepG-2) and the inhibitory impact has been evaluated. With effective concentrations of 66.78 ± 4.76 μM, the CHB gave a respectable effect to prevent 50% of cell multiplication. According to the virtual screening (molecular docking study), the ligand and complex exhibited binding affinity values of −7.4 and −7.9 kcalmol^–1, respectively. As a result, the chosen compounds may be used as potential medications to treat human liver cancer in the future. The work in this direction is in progress.

The original contributions presented in the study are included in the article/Supporting Information; further inquiries can be directed to the corresponding author/s.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c09673.

• Experimental section (materials and procedures, assessment of cytotoxicity, syntheses of CHB and Mn­(II)–CHB, molecular docking (MD) and DFT details); the mass spectrum of CHB (Figure S1); the proton and ¹³C NMR spectra of CHB (Figures S2 and S3); the FT–IR spectra of CHB and Mn­(II) complex (Figure S4); the electronic spectra of CHB and Mn­(II) complex (Figure S5); the XRD graph of the ligand and Mn­(II) complex (Figure S6); the variation of optical parameters (band gap energy, refractive index, optical conductivity, and penetration depth) for the ligand and Mn­(II) complex, Figures S7–S10; and the interaction and binding scores of CHB and Mn­(II) complex with 1i7i (HCC) receptor protein (Table S1) (PDF)

The authors declare no competing financial interest.

Published as part of ACS Omega special issue “Energy Storage across Scales”.